1. Field of the Invention
The present invention relates to a method of reducing corrosion of a metal material for restraining corrosion of a material to be exposed to high temperature water, for example, constituting structural members or elements of a nuclear power plant or a thermal electric power plant.
2. Description of the Related Art
As a metal material structure to be exposed to high temperature water, a boiler of a thermal electric power plant, a pipe or duct to be contacted with a hot water generated thereby will be presented. Moreover, in a case of a nuclear power plant, a nuclear reactor structural material and a peripheral pipe or duct thereof will be presented.
As a corrosion preventing measure for the pipe or the structural material, or the like, a countermeasure according to water quality improvement to the high temperature water has actively been executed. For example, in the thermal electric power plant, there has been provided a corrosion reducing counter measure to control the water quality by adding a pH adjuster, a deoxidant or the like.
In contrast, in a boiling water reactor power plant (hereinafter abbreviated as the BWR power plant), oxygen, hydrogen peroxide or the like generated by radiolysis of the water in a radiation field exist in the nuclear reactor water. Furthermore, it is also known that a stainless steel and a nickel-based alloy as the nuclear reactor structural material generate stress corrosion cracking under the existence of the oxygen and the hydrogen peroxide in the high temperature environment as like in the nuclear reactor.
For the countermeasure, a hydrogen injecting technique for injecting a hydrogen from a feed water system to reduce the oxygen and the hydrogen peroxide has been executed in several nuclear power plants in various countries. The effect of reducing the oxygen and the hydrogen peroxide appears as reduction of the corrosion potential of the material.
Generation of the stress corrosion cracking and progress of the cracking depend on the corrosion potential. With a lower potential, generation of the cracking and progress of the cracking can be restrained. According to such background, the hydrogen injection is executed, however, a harmful influence is involved in that the dose rate of the turbine system rises.
This is because an element N16 produced at a nuclear reaction reacts with the hydrogen so as to provide a volatile ammonia which is easily moved to a steam turbine or like system. In addition, various kinds of equipment are required for injecting an oxygen for re-combining an excessive hydrogen in the off gas system generated by the injected hydrogen or the like.
In order to reduce a harmful influence to a minimum degree and lower the corrosion potential of the structural material, recently, there has been proposed a method of adding a noble metal to the nuclear reactor water and adhering the noble metal to the structural material to lower the corrosion potential by injecting the hydrogen by a small amount. This is a method of utilizing the nature of a noble metal such as a platinum of selectively scavenging reversible reaction of the hydrogen of a lower potential. By adhering the noble metal to the structural material, the corrosion potential can be lowered by injecting the hydrogen by a small amount.
However, according to the method of adhering the noble metal to the structural material described above, the hydrogen reaction is an important factor and addition of the hydrogen is inevitable, a large load is applied to the equipment. Thus, it is necessary to solve problems in terms of the equipment. Furthermore, in the case of executing the method of adding the noble metal to the nuclear reactor water in the actual plant, since the noble metal adheres also to the fuel assembly, in particular, to a zirconium oxide film of the fuel material such as fuel cladding tube or pipe, it becomes anxious to increase the oxidization and the hydrogenation of the fuel material.
Since an influence of the above fact will cause an adverse effect to improvement of fuel burnup, it is important to reduce the addition amount of the noble metal as well as reduce the use amount of the expensive noble metal or to develop a substance to be substituted for the noble metal.